Plasmas, or ionized gases made up of excited atoms, ions, radicals and electrons, are widely used for modification of surfaces without affecting bulk properties of samples. Plasma treatments have been developed and used for more than 30 years in the microelectronics and textile industries, biomedical, environmental, materials and chemical process engineering fields, as well as in surgery. Plasmas offer a high-density source of energy and/or reactive species.
Atmospheric-pressure plasmas are used in a variety of materials processes. Traditional Atmospheric-pressure plasma sources include transferred arcs, plasma torches, corona discharges, and dielectric barrier discharges.
Plasmas are generally classified as thermal or non-thermal. Thermal plasmas are characterized by a high energy density and high gas and electron temperatures, which are nearly equal in value. Thermal plasmas are used where a high enthalpy source is required.
Non-thermal plasmas are characterized by a low energy density, and a significant difference between the temperature of the heavy species (the “gas”) and the electrons. Non-thermal plasmas are chemically selective, more energy efficient and represent a lower thermal load to processed materials and surfaces. Non-thermal plasmas are well suited for the treatment of biological and biocompatible surfaces.
Non-thermal plasmas can be produced in controlled environments and under reduced pressure conditions. Non-thermal plasmas can also be produced under atmospheric pressure conditions in open-air configurations or controlled atmosphere conditions.
Non-thermal plasma sources come under various shapes for various applications, including large volume or large area plasma sources for bulk or large surface-area treatments, respectively, and plasma torches or jets for remote exposure applications.
The plasma-forming zone of the non-thermal plasma source, in the case of uniform plasma, is called the glow. The region of plasma extinction where the plasma species de-excite and recombine is called the afterglow.
The interest in atmospheric pressure non-thermal plasma sources for the modification and treatment of biological and biocompatible surfaces has grown considerably over the last decade. Such sources offer convenient means for sterilization (see for example: Laroussi M, Mendis D A and Rosenberg M 2003 New J. Phys. 5 41.1-41; Roth J R, Sherman D M, Ben Gadri R, Karakaya F, Chen Z, Montie T C, Kelly-Wintenberg K and Tsai P P-Y 2000 IEEE Trans. Plasma Sci. 28 56-63); surface functionalization (see for example: Bruil A, Brenneisen L M, Terlingen J G A, Beugeling T, Van Aken W G and Feijen J 1994 J. Colloid Interface Sci. 165 72-81), cell removal (see for example: Stoffels E, Kieft I E, Sladek R E J 2003 J. Phys. D: Appl. Phys. 36 2908-2913), microcontact printing of protein onto polymer substrates (see for example: Schmalenberg K E, Buettner H M and Uhrich K E 2004 Biomaterials 25 1851-1857) and tissue modification.
The development of such plasma sources is not without challenges. The main challenges include for example 1) the difficulty to sustain a stable and uniform glow discharge over large surface areas in film deposition and surface functionalization; 2) the need to maintain a high degree of non-thermal equilibrium to minimize the thermal load to the substrates of interest, while maintaining a high degree of chemical reactivity and 3) the extremely rapid recombination of the reactive species in the plasma afterglow when a torch configuration is used.
Several devices have been developed to produce relatively small non-thermal plasma streams at atmospheric pressure. A miniature inductively-coupled plasma (ICP) torch using an argon/halogen mixture was recently developed for localized and high rate etching of silicon wafers (see for example: Ichiki T, Taura R, Horiike Y 2004 J. Appl. Phys. 95 35-39). Miniature capacitively-coupled plasma torches using mixtures of He and halogen gases, or oxygen, have also been developed for local etching of silicon (see for example: Koinuma H, Ohkubo H, and Hashimoto T, 1992 Appl. Phys. Lett. 60 816-817) and etching of polyimide (see for example: Jeong J Y, Babayan S E, Schutze A, Tu V J 1999 J. Vac. Sci. Technol. A: Vac. Surf. Films 17 2581-2585). Other capacitively-coupled plasma torches have been developed for the treatment of heat sensitive materials (Park J, Henins I, Herrmann H W and Selwyn G S 2001 J. Appl. Phys. 89 20-28) and as a source of active species for the depletion of contaminants present in liquid hydrocarbons (Guerra-Mutis M H, Pelaez U C V and Cabanzo H R 2003 Plasma Sources Sci. Technol. 12 165-169). Single electrode configurations were also reported for silicon oxidation, synthesis of carbon nanostructures (Kikuchi T, Hasegawa Y and Shirai H 2004 J. Phys. D: Appl. Phys. 37 1537-1543) and removal of photoresist (Yoshiki H, Taniguchi K and Horiike Y 2002 Jpn. J. Appl. Phys. 44 5797-5798). There have been other miniature plasma sources developed for remote analytical systems, such as the microwave plasma torch (MPT), used as an excitation source for atomic spectroscopy (see for example: Jin Q, Zhu C, Borer M W, Hieftje G M 1991 Spectrochim. Acta B 46 417-430; Stonies R, Schermer S, Voges E and Broekaert J A C 2004 Plasma Sources Sci. Technol. 13 604-611; Bilgic A M, Prokisch C, Broekaert J A C, Voges E 1998 Spectrochim. Acta B 53 773-777).
These plasma sources share a number of common characteristics, including: 1) a high-frequency excitation (RF or microwave, except for Guerra-Mutis M H, Pelaez U C V and Cabanzo H R 2003 Plasma Sources Sci. Technol. 12 165-169), which favors the formation of a non-thermal plasma at atmospheric pressure under low-voltage excitation conditions (few hundred volts); 2) use of He or Ar as the main plasma-forming gas, 3) use of minute amounts of an additional gas as the source of reactive species, and 4) a configuration permitting the rapid transport of excited species to the surface of interest in remote exposure applications.
In the field of local bio-applications, atmospheric pressure plasma sources have been scarcely reported since the early 1990's. An argon plasma coagulation (APC) device, which uses a small rod as the powered electrode and the patient as the ground electrode, was commercialized as a small-scale electrocoagulation tool (Storek D, Grund K E, Gronbach G, Farin G, Becker H D 1993 Z Gastroenterol. 31 675-679 (in German)). It was demonstrated through clinical trials that the APC caused significantly less damage to tissues than YAG lasers. The APC produces an electrical discharge between the electrode and the lesion, which desiccates, coagulates, and devitalizes through heat effects (see Letard J C 2000 Acta Endoscopica 30 (S2) 414-415; Schreiber J, Hofman B, Schumann H J and Rosahl W 2000 Respiration 67 287-290).
Recently, the treatment of biological tissue and cells (Stoffels E, Kieft I E and Sladek R E J 2003 J. Phys. D: Appl. Phys. 36 2908; Kieft I E, Darios D, Roks A J M and Stoffels E 2005 IEEE. Trans. Plasma Sci. 33 771; Fridman G, Peddinghaus M, Fridman A, Balasubramanian M, Gutsol A and Freidman G 2005 Proc. 17th Int. Symp. on Plasma Chemistry (Toronto)), as well as the functionalization of surfaces to control cell adhesion, have been investigated (De S, Sharma R, Trigwell S, Laska B, Ali N, Mazumder M K and Mehta J L 2005 J. Bio mater. Sci. Polym. Ed. 16 973-989; van Kooten T G, Spijker H T and Busscher H J 2004 Biomaterials 25 1735-1747). Non-thermal, atmospheric pressure plasma sources are particularly suitable for use with heat-sensitive substrates. Having the bulk temperature of the plasma close to room temperature reduces the negative effects of thermal loads on such materials as human tissues and biodegradable polymers used in the construction of biomedical devices, while still being able to take advantage of the highly reactive nature of the plasma.
The treatment of biomaterials with non-thermal plasmas has been widely researched, and various technologies are used commercially in the modification of tissue culture vessels. Cell attachment is enhanced by modifying the culture dish surface using plasmas of various gas compositions to increase the amount of oxygen and/or nitrogen groups incorporated into the surface (Corning Incorporated 2005 Corning cell culture selection guide (New York: Corning); BD Biosciences 2001 BD Falcon (TM) cell culture products (Bedford: Beckton, Dickson and Company). The modifications enhance the hydrophilicity of the surface by the addition of polar groups, and increase cell adhesion. At the laboratory level, researchers have investigated plasma treatment to micropattern surfaces to study neuronal networks (Brown I G, Bjornstad K A, Blakely E A, Galvin J E, Monteiro O R and Sangyuenyongpipat S 2003 Plasma Phys. Control. Fusion 45 547-554), the fabrication of biosensors and the imitation of in-vivo cell patterning on implants to improve biocompatibility.
Currently, most plasma patterning is done using photolithographic techniques. A chemical coating is hardened with UV light through a laser-cut metal mask, and the unaffected areas are washed clean. The whole surface is plasma treated, functionalizing the areas not covered with the resist layer (Ohl A and Schrader K 1999 Surf. Coat. Technol. 116-119 820-830). The resist is removed, and the functionalized pattern is left on the surface. There are a few aspects of photolithography that inhibit its use on biomaterials. First, it has traditionally been used on glass or silicon surfaces, and the chemicals used in the process can accelerate the degradation of the polymers used as biomaterials (Miller C, Shanks H, Witt A, Rutkowski G and Mallapragada S 2001 Biomaterials 22 1263-1269) and introduce a source of contamination in cell culture. Secondly, masks are costly, and give only one pattern. Moreover, the masking process does not work well on curved surfaces. Schroder et al. (Schroder K, Meyer-Plath A, Keller D and Ohl A 2002 Plasmas and Polymers 7 103-125) have been successful in plasma micropatterning directly through a mask without using the chemical resist, however, they found that the mask was sensitive to handling and heat. The use of a miniature plasma source that is capable of 3-D movement could circumvent the present difficulties of micropatterning on unsymmetrical, biodegradable surfaces.
In addition to surface patterning, the possibility to perform tissue/cell treatment is of tremendous interest. A so-called plasma needle has been previously used to treat mammalian cells (Kieft I E, Broers J L V, Caubet-Hilloutou V, Slaaf D W, Ramaekers F C S and Stoffels E 2004 Bioelectromagnetics 25 362-368). At a power level of 0.1-0.3 W, Kieft et al showed cells could be detached and would reattach within four hours. The important role of media coverage was highlighted; too little and the cells dehydrated and died, and too much and the reactive species from the plasma did not reach the cell (Kieft I E, Darios D, Roks A J M and Stoffels E 2005 IEEE. Trans. Plasma Sci. 33 771). It was proposed that the adhesion molecules, both those responsible for cell-cell and cell-substrate binding, had been interrupted based on the visual inspection of the behaviour of the cells after treatment, and viability stains. Other potential oxidative effects on the cell due to plasma treatment include lipid peroxidation (the deterioration of the cell membrane due to the oxidation of the lipids), protein oxidation and cell death due to an imbalance of reactive oxygen and nitrogen species (ROS and RNS).
Technologies currently used to permeabilize cells include capillary microinjection, surfactants and electroporation, whose primary limitations are low throughput, cell death, and the need for cells in suspension, respectively. Inducing cell death restricts time dependent studies, while the trypsinization required to produce a cell suspension disrupts cell adhesion proteins, limiting the study of certain cell processes.
The present invention seeks to meet these needs and other needs.